The invention relates to the scrambling concealment of a video or other time-domain information signal when displayed on a monitor or screen, to prevent unauthorized viewing thereof and, more particularly, to a technique for increasing the effectiveness of a horizontal sync overlay signal by selectively modulating a specialized sync pulse of the signal, to thereby maximize the concealment when the signal is displayed on any type of television set. The invention also includes an associated technique for demodulating the scrambled signal to provide a descrambled, viewable video signal when authorized.
There are various techniques used at present to provide scrambling concealment, and to provide descrambling, of a time domain information signal such as video signals. The purpose of such techniques is to prevent the unauthorized use of the signals. In the case of a video type of information signal, the unauthorized use which is prevented is the visual display of the signals for purposes of entertainment or instruction. Such signals are vulnerable to unauthorized use, for example, when they are broadcast over a satellite or microwave link, or when transported in the form of recordings on video tape, discs, and the like. It follows that it is highly desirable to prevent unauthorized users from successfully intercepting transmitted signals or from successfully viewing recorded signals.
To achieve the desired optimum results, a scrambling concealment technique must at least meet two conditions. First, the technique must always provide total and thus effective concealment of displayed pictures and, second, the technique must always work, that is, must always securely effect total concealment in all concealment situations and regardless of the types of monitors or television sets being used to view the signal.
Typical of present techniques for scrambling video signals to provide concealment thereof are those techniques known for example as video signal inversion, horizontal sync pulse suppression, interfering signal superimposed on the program video, video time line shifting or "wobbling", etc.
In the technique of video inversion, the video signal is "mapped" from white-to-black and vice versa or, in a color video signal, from a given color to its complementary color. Video inversion offers some concealment of the television picture but is ineffective, for example, for program material containing text.
Sync suppression of the horizontal sync pulses (H sync) is reasonably effective but the extent of its concealment is dependent upon scene content such as, for example, the average picture level. If the video inversion and/or sync suppression techniques are used together, the concealment may be less effective, especially when the techniques are used with "letter boxed" movies, because the properties of the inverted video in letter boxed movies may cause the pertinent circuits of the television set to use the stable letter boxed portion of the inverted video signal (for example, the top and bottom portions of the screen) rather than the unstable sync signal to provide a horizontal sync for the set. The properties of "letter boxed" movies are further explained below.
The technique of adding an interfering signal to the video signal to conceal it may be effective, but unscrambling the concealed video signal is very difficult. Thus the technique is impractical generally due to the resulting small nonlinearities generated throughout the television transmission and reception system.
A disadvantage of the line time shifting technique is that the scrambled signal is not completely concealed when viewed on a television set. That is, while very annoying, the resulting horizontally jumping picture still can be watched and partly understood by a determined viewer, whereby the technique does not provide an adequate level of concealment for all program material.
A majority of the above mentioned disadvantages of the various scrambling concealment techniques discussed above are overcome by, for example, the scrambling technique disclosed in the application Ser. No. 08/400,831, incorporated by reference, of previous mention. In this technique, the horizontal sync signal is time shifted, or "wiggled", to add a concealment overlay to the main time shifting scrambling system. That is, there is provided a further modification of the video signal, which obscures the picture when viewed prior to descrambling to thus prevent an unauthorized user from seeing a recognizable picture. A typical horizontal sync overlay scrambling technique includes an embodiment which involves time shifting the location of the horizontal sync pulse in each line of video in a pseudo random fashion from line to line. In a second embodiment, the sync pulse is time shifted in more complex fashion by the application of two genuinely random shifting patterns to thereby wiggle the sync pulse location from line to line in the more complex fashion.
This typical horizontal sync overlay scrambling technique likewise experiences various disadvantages. It was found that with some types of television (TV) sets, the wobbling effectiveness was related to the wobbling (modulating) frequency and to the wiggled horizontal sync pulse width and, of course, to the amount of sync pulse positional displacement. For example, since the standard horizontal sync pulse is about 4.7 microseconds (.mu.s) in width and occurs during a standard 10.9.+-..2 .mu.s horizontal blanking interval, there is a decided limitation on the degree of time shift that can be applied to the horizontal sync pulse on successive lines. For example, the available time shift may comprise.+-.1.5 .mu.s. It follows therefore that there is a decided limitation on the corresponding amount of positional displacement and thus of concealment that is imparted to the scrambled picture.
To get maximum effectiveness it would appear that using a horizontal sync pulse of very narrow width of about 2 to 2.3 .mu.s will allow for the maximum wobbling displacement within the horizontal blanking interval and thus provide a corresponding maximum concealment of the signal. However, with more modern TV sets, for example the multiple television standard sets in existence today which employ very high speed horizontal automatic frequency control (AFC) circuits, a very narrowed wobbling horizontal sync can be construed by the pertinent AFC circuit of the TV sets as noise and thus ignored. The whole purpose of the horizontal sync overlay concealment is to have the modified horizontal sync pulses, that purposely are modulated, consistently be sync separated in the television set so that the AFC horizontal phase lock loop (H PLL) in the set always will lock onto the wobbling sync pulse to cause concealment of the signal when it subsequently is displayed on the TV set. It was found however, that if the modulated horizontal sync pulses are too narrow, the television set conceals very little due to the fact that the H PLL does not lock onto the very narrow sync pulses.
Some concealment is made possible when using the very narrow horizontal sync pulses, but these may have to be modulated at a substantially lower frequency. To illustrate the problem, in the instance where a television set locks onto the wobbling sync pulse and provides full concealment, the modulating frequency may be as high as 1000 Hz or higher. In a situation where a TV set construes the wobbling horizontal sync pulse as too narrow and doesn't lock onto the modulating frequency, minor concealment will occur if the frequency used is about 100 Hz to 1000 Hz. Even in this latter situation more of the concealment can be recovered by lowering the frequency of the horizontal sync position modulation to less than about 20 Hz (3 Hz to 10 Hz typical).
Thus, the above technique is not an optimal solution for increasing concealment for the types of TV sets of previous mention which don't respond well to very narrow wiggled horizontal sync pulses. Further, lowering the modulating frequency to recover some concealment means that small text or information on screen is not well concealed, because a higher frequency for the wiggled horizontal sync pulses is desirable when attempting to conceal text.
Accordingly, the problems of the scrambling concealment techniques first enumerated above, may be circumvented by the horizontal sync overlay technique discussed immediately above, wherein the horizontal sync pulse is selectively time shifted, but the width of the horizontal sync pulse is narrowed to on the order of 2 to 2.3 .mu.s. The resulting available time shift, or pulse position modulation, within the horizontal blanking interval is of the order of .+-.2 .mu.s which in most instances provides effective concealment of, for example, text in a scrambled video program signal. However, in a situation where the video inversion technique of previous mention is used alone or in combination with other scrambling techniques in concealing letter boxed movies or programs, the very narrow width horizontal sync pulse overlay technique proves to be inadequate when the movies are played on the certain types of television sets of previous discussion. Such television sets, including the multiple television standard types of sets mentioned above, respond to the letter boxed portion of a video signal as if it were the horizontal sync signal, rather than detecting and thus responding to the very narrow width horizontal sync pulse. That is, the wiggled horizontal sync pulse of narrowed width is not detected by the sync separator circuit of the television set when displaying the video signal. Instead, the sync separator circuit locks onto a stable portion of the letter boxed video signal, as is further described below. It follows that under these conditions, the scrambling horizontal sync overlay technique using a wiggled, very narrow sync pulse fails to consistently provide effective concealment of the corresponding displayed video signal.
Accordingly, it is highly desirable to provide a scrambling concealment technique in which effective concealment is assured in every situation and regardless of the type of television set used in an attempt to display the scrambled video signal.